The invention relates to an acoustic transducer in accordance with the features stated in the preamble of the claim.
In the document, xe2x80x9cHiroshi Nishiyama et al. Piezoelectric Sound Components Used in a Broad Range of Applications, JEE Journal of Electronic Engineering, Aug. 1, 1988, pages 62-66, XP000570731,xe2x80x9d such an electroacoustic transducer is disclosed, which is configured as a monomorphous flexural vibrator. Also, in the document, xe2x80x9cYukata Ichinose Optimum Design of a piezoelectric diaphragm for telephone transducers, Journal of the Acoustical Society of America, Vol. 91, No. 1, Jan. 1, 1992, pages 1246-1252, xp000231994,xe2x80x9d a modeling of such a flexural vibrator is disclosed. Lastly, a flexural vibrator with a pot-like housing is disclosed in U.S. Pat. No. 5,636,182.
Electroacoustic transducers, especially air electroacoustic transducers, serve to convert electromagnetic waves to mechanical waves or vice versa. On the surface of the electroacoustic transducer or ultrasound transducers the greatest possible particle movement and fast rise times are sought after. Ultrasonic technology is based on electroacoustical waves, i.e., mechanical waves, and a wave of this kind builds up from vibrations of the individual particles in the medium in which it is propagated. In fluids, i.e., gases and liquids, no transverse waves occur, so that only the longitudinal or compression waves are of interest. The intensity of a wave of this kind follows the formula:
I=0.5xc3x97Zxcfx892"xgr"
In this formula, Z represents the electroacoustic impedance of the propagation medium (product of density and sound velocity), xcfx89 the particle frequency, and "xgr" the particle deflection. Furthermore, for compression waves there is the relationship:
Z=p/c
with the electroacoustic impedance Z, the sound velocity C and the sound pressure p. Starting out from air as the propagation medium (Z=0.430MRayl) it is apparent that the amplitude of the particle deflection in comparison to its force determines intensity.
Different principles are known for the conversion of electrical energy to mechanical under the marginal condition of sound radiation in gases. Thus, a thickness vibrator consists of a piezoelectric ceramic in the form of a cylinder or a disk. It vibrates piston-like in its thickness, the thickness determining the resonance frequency as a geometrical factor. By varying the diameter it is possible to influence the spatial distribution of the forwardly emitted sound field.
Often these vibrators are provided on the front side with acoustically optimized xcex/4 coatings or damped on the reverse side with suitable materials in order to achieve a better transfer ratio. A chief advantage in this technique is the high transmission bandwidth that can be achieved (mechanical qualityxc3x9710). A problem is the thickness of the piezoceramic that is necessary at low frequencies which call for a high electrical source and load resistance.
Furthermore, flexural vibrators are known which are distinguished by a sandwich structure, and a distinction is made between a monomorphous flexural vibrator and a bimorphous flexural vibrator. The monomorphous flexural vibrator consists of a diaphragm (usually metal) onto which the piezoceramic is applied. The ceramic is smaller than the diaphragm diameter. Since the ceramic is operated in a planar resonance, its radius influences the resonance frequency. Thus the thickness of the ceramic can be very small, and the electrical source resistance can be low. The resonance frequency is determined by the geometry of the individual components and their adhesion to one another. The transducers are very inexpensive, very efficient and small, but they have an extremely narrow band (relative 6 dB P/E bandwidthxc3x973%). In the case of the additional damping of such vibrators the efficiency decreases extremely. On the other hand the bimorphous flexural vibrators are hard to operate at frequencies above 80 kHz and are relatively expensive.
Lastly, electrostatically operated transducers are known in which the deflection of a diaphragm is produced by electrostatic forces. Such transducers react very sensitively to variations in the ambient parameters, such as temperature and humidity, and are relatively expensive.
With the standard techniques explained, on the one hand very narrow-band and effective air electroacoustical transducers can be created, and on the other hand wide-band but quite insensitive air electroacoustical transducers can be made.
Setting out from this knowledge, the invention is addressed to the problem of proposing an electroacoustic transducer, especially an air electroacoustic transducer, with which an improved, efficient conversion of electromagnetic waves to mechanical waves, or vice versa, can be achieved. The electroacoustic transducer is to combine simple construction with high reliability of operation, and is to require low manufacturing costs. A wide-band air electroacoustic transducer is to be created which has improved sensitivity.
The solution of this problem is achieved in accordance with the characteristics stated in claim 1.
The electroacoustic transducer according to the invention combines in an especially advantageous manner two vibrator principles. A composite of a piezoceramic disk and a diaphragm, preferably made of a mixture of epoxy and hollow glass spheres or an acoustically comparable material which forms a monomorphous flexural vibrator, is provided. The diaphragm is preferably part of a transducer housing. Furthermore, the planar vibration mode in the piezoceramic is converted by means of the cross-contraction ratio to a thickness vibration which, after transformation by a coupling coating layer, which has a low acoustical impedance, is matched to the propagation medium, preferably air. Further refinements and special embodiments of the invention are given in the dependent claims as well as in the description that follows.